1. Field of the Invention
The invention relates to a membrane electrode assembly and a polymer electrolyte membrane fuel cell. More specifically, the invention relates to a polymer electrolyte membrane fuel cell preferable for use as an on board power source, a stationary small generator, or a cogeneration system or the like, and a membrane electrode assembly for use in such a polymer electrolyte membrane fuel cell.
2. Description of the Related Art
A polymer electrolyte membrane fuel cell is based on units of membrane electrode assemblies (MEA) in which electrodes are joined to both sides of a solid polymer electrolyte membrane. Also, in the polymer electrolyte membrane fuel cell, electrodes typically have a bilayer structure of a diffusion layer and a catalyst layer. The diffusion layer is for supplying reaction gas and electrodes to the catalyst layer and is made of carbon paper or carbon cloth or the like. Also, the catalyst layer is the portion that serves as a reaction field for the electrode reaction, and typically formed of a composite body of carbon which carries an electrode catalyst such as platinum, and a solid polymer electrolyte membrane (an electrolyte in the catalyst layer).
An all perfluorocarbon electrolyte (i.e., an electrolyte which does not include a C—H bond in the polymer chain) having high oxidation resistance is typically used for the electrolyte in the catalyst layer or the electrolyte membrane that forms this kind of MEA. Examples of such an electrolyte include Nafion (registered trademark; manufactured by DuPont), Aciplex (registered trademark; manufactured by Asahi Kasei (Corp.)), Flemion (registered trademark; manufactured by Asahi Glass Co., Ltd), etc. Also, an all perfluorocarbon electrolyte is highly resistant to oxidation but is also generally extremely expensive. Therefore, in order to reduce the cost of the polymer electrolyte membrane fuel cell, the use of a hydrocarbon electrolyte (i.e., an electrolyte which include a C—H bond but not a C—F bond in the polymer chain) and a partial perfluorocarbon electrolyte (i.e., an electrolyte including both a C—H bond and a C—F bond in the polymer chain) is also being considered.
However, in order to put a polymer electrolyte membrane fuel cell into practical use as an on board power source, for example, some problems still have to be solved. For example, while a hydrocarbon electrolyte is less expensive than an all perfluorocarbon electrolyte, it also tends to be degraded by peroxide radicals. Also, a gasket is usually secured around the outer peripheral portion of a solid polymer electrolyte membrane to ensure a gas seal. However, constriction by this gasket applies shear stress to the solid polymer electrolyte membrane which leads to failure of the membrane and reduces the gas seal function. Furthermore, the solid polymer electrolyte membrane repeatedly expands and contracts due to changes in temperature and humidity that occur during use. Therefore, stress is apt to occur particularly at the outer peripheral portion of the electrode.
There have been various proposals to solve the problem described above. For example, Japanese Patent Application Publication No. JP-A-2004-018573 (reference 1) describes a proton-conducting polymer membrane obtained by immersing a sulfonated polyphenylene sulfide film in a solution in which magnesium acetate tetrahydrate, calcium acetate, aluminum triisopropoxide, or lanthanum nitrate hydrate has been dissolved. This reference cites that i) some of the hydrogen atoms of a proton-conducting substituent that includes a sulfonated polyphenylene sulfide film is substituted with metal ions such as Mg2+ by applying this kind of treatment, and ii) oxidation resistance of the sulfonated polyphenylene sulfide film is improved by substituting some of the hydrogen atoms with metal ions.
Also, Japanese Patent Application Publication No. JP-A-2000-215903 (reference 2) describes a membrane electrode assembly in which fibers of a center portion of plain-woven fabric woven with polytetrafluoroethylene fibers are cut in chained lines and an open portion is formed in the center portion by pulling out the cut fibers. A composite membrane is then made by impregnating the entire plain-woven fabric with Nafion solution, and an electrode having a larger area than the open portion is joined to both sides of this composite membrane. This reference cites that it is possible to inhibit the membrane from breaking due to the constriction by the gasket, or inhibit degradation of the membrane due to the portion to the inside of the gasket and to the outside of the electrode expanding and contracting.
One issue with fuel cells is how to suppress degradation of the electrolyte membrane. Two root causes of degradation are the decomposition of polymer molecules due to chemical species such as radicals (i.e., a chemical cause) and damage of the membrane from stress (i.e., a mechanical cause).
According to reference 1, when substituting some of the hydrogen atoms of the proton-conducting substituent with a given species of metal ion, the chemical resistance of the electrolyte membrane increases. However, depending on the ion species that is being ion-exchanged, sufficient chemical resistance for practical use is unable to be obtained. Also, the method described in reference 1 performs ion exchange with the entire membrane so some of the protons must be left in order to maintain power generating performance. As a result, the effect is reduced by half compared with a case in which ion exchange is performed with all of the protons. Further, among the ion species specifically described in reference 1 are some which are effective for improving chemical resistance, however, they are not very effective for improving mechanical strength.
On the other hand, the method described in reference 2 is effective for suppressing degradation of the membrane caused by constriction from the gasket and expansion and contraction of the membrane. However, this method does not enable degradation of the membrane due to a chemical cause to be suppressed. Also, the outer peripheral portion of the membrane is combined with polytetrafluoroethylene or the like so there is a possibility that the membrane and the reinforcing member may separate during use.
Moreover, degradation of the membrane due to a chemical cause starts at a specific portion of the membrane. Also, in order to suppress degradation of the membrane due to both the chemical and mechanical causes, the moisture content of the membrane must also be controlled. However, there is no related art that proposes a method to strengthen an electrolyte membrane that takes these points into account.